It is an object of the invention to make such a circuit arrangement which is known from United Kingdom patent specification No. 1,245,552, also suitable for different free-running frequencies. It is then possible to bring the natural frequency, i.e. in the absence of synchronizing pulses to substantially the nominal value. When the switching arrangement is for example used for deflection purposes in a television receiver, it is in this way prevented that the displayed picture runs rapidly so that it would not be viewable. When, however, the synchronizing pulses occur, the natural frequency has to be reduced and direct synchronization has then to be effected, so that a perfectly stationary picture is obtained. The amplitude or the energy content or the direct current component of the separated synchronizing pulses may be used for such a change-over.
Different television standards have different deflection frequencies. Particularly as regards the field deflection, the European television standard uses 50 Hz, consequently a period of 20 ms, whereas the United States television standard uses 60 Hz, which corresponds to a period of 16.67 ms. For receivers which must be suitable for use in a television receiving area wherein both standards are used, a change-over is, consequently, required. In south Germany it is, for example, possible to receive television signals of both standards in the same location, as there United States troops operate transmitters which operate on the basis of the NTSC system with a field deflection of 60 Hz.
Conventional television receivers do not require a special circuit arrangement for the deflection frequency as the line frequency is approximately the same for both standards. In the case of direct synchronization, the 60 Hz field frequency pulse arrives somewhat earlier, but it introduces a normal retrace. As, however, this synchronization occurs at an earlier instant on a rising sawtooth edge and, consequently, at a lower amplitude, it causes the amplitude of the field deflection to be reduced according to the ratio of the periods by approximately 17%, so that the picture geometry is correspondingly distorted.
A further object of the invention is to render it possible that, at a different frequency of the synchronizing signal, the sawtooth generator can be switched thus that the sawtooth amplitude remains substantially constant.